Grid does nothing with any boss frames, including the "original" Blizzard boss frames. If you are not seeing boss frames, you have some other addon -- not Grid -- hiding them. Many general-purpose unit frame addons like PitBull, Stuf, and Shadowed Unit Frames include options to hide boss frames.

If you are asking to have boss frames added to Grid, that is not possible. See the recent posts in the WowAce forum thread, and this previous ticket: